Autochthonous Versus Allochthonous Organic Matter in Recent Soil C Accumulation Along a Floodplain Biogeomorphic Gradient: An Exploratory Study

The mechanisms controlling soil succession in floodplains remain much less studied than in uplands due to the complexity that flooddriven erosion and sedimentation bring into soil development processes. The amount of organic matter and C generally grows with soil ageing and is controlled by multiple and interacting allogenic and autogenic factors, but to what extent the production of organic matter by in situ vegetation contributes to soil formation in floodplains remains unknown. The objective of this work was to explore the importance of autochthonous organic matter versus allochthonous organic matter in organic C accumulation of floodplain forest soils along a vegetation succession and hydrogeomorphic connectivity gradient. Physicochemical analyses of sediment collected after one single flood event in a large Mediterranean floodplain (Middle Ebro, a 9th order regulated river reach in NE Spain) were used to estimate the proportion of organic C found in the topsoil (first 10 cm) samples of young (50 yr) floodplain forests that had an allochthonous (i.e., % of organic C deposited by floods) or autochthonous (i.e., % of organic C produced in situ by vegetation) source. Results of this exploratory study showed that the accumulation of autochthonous organic C in the floodplain topsoil only occurred in floodplain forests older than 50 year-old, but even then, it was more than six-fold less abundant than that with an allochthonous origin. Moreover, a linear mixed effect model showed that, although autochthonous organic C accumulation was mainly explained by the forest structure, a small proportion of it was also controlled by an allogenic factor, the groundwater table depth. Then, groundwater table depth variations could be partly controlling autochthonous organic matter production and decomposition in this Mediterranean floodplain. Although flow regulation and embankment has dramatically limited the hydrogeomorphic dynamics of the river, allogenic overbank sedimentation during flood events still controls floodplain soil succession and organic C accumulation in the floodplain.